Blender viewport lags while animating
This is a common problem, the more complex is the mesh, the armature, the weights, the more the animation gets slow.

Youn can use various strategies to check your animations:
1 - hide (maybe using layers, even separating some vertices of the mesh) all vertices you can
2 - building a fake simpler mesh, which will be very fast if done using bone parents instead of weighted vertices: i.e. it's very common to build a characetr using the face as it is and using simple cubes or cyliders for torso, arms, legs, parenting each whole object to its corrispondent bone: in this way you avoid lots of calculations.
3 - making a superfast OpenGL render of the scene, which can be analyzed always running in perfect realtime.

Maybe others users know some more strategies.